1. Field of Invention
This invention relates generally to biosensors for measuring physiological analytes in humans, and particularly to biosensors suitable for implantation to provide in vivo monitoring of a selected analyte, such as monitoring of blood glucose levels in diabetics.
2. Description of Related Art
For some time the medical community has recognized a need for implantable biosensors to monitor physiologically important analytes. The need is particularly pressing for continuous monitoring of blood glucose in diabetics, since failure to properly maintain glucose levels leads to serious consequences in both the short and long term. The long-term consequences include kidney failure, blindness, and amputation. To date, however, the only test readily available is a fingerstick kit, which does not provide continuous monitoring. Most diabetics use such kits infrequently at best, because of the pain and inconvenience associated with them.
In developing various implantable devices, hydrogels have been widely used as protective biocompatible coatings for the devices. Hydrogels are generally defined as polymeric materials that swell in water and other fluids, absorbing the fluid within the polymer network without dissolving. Hydrophilic hydrogels have a large amount of water content at equilibrium, and good biocompatibility.
The above-described properties of hydrogels make them attractive for use in implantable biosensors. One such device is an implantable amperometric sensor intended to measure glucose levels in blood or body fluids (U.S. Pat. No. 4,703,756 to Gough et al.). A second type of hydrogel biosensor which uses a pressure transducer to measure changes in osmotic pressure in pH-sensitive hydrogels, developed by the present inventors, is described in U.S. Pat. No. 6,268,161 to Han et al., and in co-pending U.S. patent application Ser. Nos. 09/308,392 and 09/644,323.
The Gough et al. biosensor uses the enzymatic activity of glucose oxidase (GOX) to measure glucose levels. GOX catalyzes the conversion of glucose to gluconic acid and hydrogen peroxide (H2O2), consuming oxygen in the process. The GOX reaction can be followed using electrochemical transducers of various kinds, but the most advanced type of device is the amperometric sensor. In the amperometric method, an electrode produces a current proportional to the diffusional flux of hydrogen peroxide to the electrode surface, or, alternatively, proportional to the diffusional flux of oxygen (O2) to the electrode surface. An increase in the surrounding glucose concentration should increase the diffusional flux of glucose into the membrane and increase the reaction rate within the membrane. The increase in enzymatic reaction rate in turn should increase the local hydrogen peroxide concentration and decrease the local oxygen concentration within the membrane. This increases the current detected by a hydrogen peroxide-based electrode sensor, or decreases the current to an oxygen-based electrode sensor. The latter approach, based on detecting the oxygen flux, requires a reference oxygen-based electrode sensor located in a hydrogel without the enzyme.
A second class of osmotic-pressure hydrogel sensors uses a pressure transducer to directly measure osmotic pressure changes in a hydrogel disposed within a rigid chamber having one open side which is covered with a flexible, semi-permeable diaphragm (Han et al., U.S. Pat. No. 6,268,161; Han et al., U.S. application Ser. Nos. 09/839,993 and 09/644,323). The pressure transducer senses changes in the pressure exerted by the hydrogel on the flexible diaphragm. Two types of such sensors have been developed. One uses pH-sensitive hydrogels having immobilized GOX. In this device, the gluconic acid produced by enzymatic action of GOX on free glucose changes the pH in the fluid matrix, causing it to swell (if the hydrogel has pendant acidic groups) or to shrink (if the hydrogel has pendant basic groups). The second type, which has potentially far wider application, uses the principles of the competitive binding assay. Both analyte and analyte-binding molecules are immobilized within the hydrogel; noncovalent bonds between the two effectively produce crosslinks. When free analyte displaces immobilized analyte, the crosslinking index changes, producing either swelling or shrinking of the hydrogel (depending on other factors in hydrogel composition). The resulting changes in osmotic pressure are measured with a pressure transducer in the same way as for the GOX osmotic-pressure biosensor. Where the analyte is glucose, the immobilized analyte binding molecule may for example be concanavalin A.
In addition to the above-described biosensors, there is another hydrogel-based glucose measurement system that measures the displacement change of the hydrogel in the pending U.S. Provisional Patent Application Ser. No. 60/316731 to Lew et al. and in co-pending U.S. patent application Ser. No. 10/054660. The swelling displacement of the implanted hydrogel is monitored by image capture from outside the body such as an ultrasound scanning device, and the change of displacement characterizes the glucose concentration.
The present invention comprises a hydrogel-based biosensor that measures the displacement of an analyte-sensitive hydrogel filament such as glucose-sensitive hydrogel filament (GSF). In order to measure the displacement, the hydrogel filament is placed in a rigid column that has at least one semi-permeable opening to permit contact between the hydrogel filament and the test fluid (a patient""s blood or other solution), and a photometric displacement detection means is provided for detecting the displacement of the hydrogel filament.
Two types of specially chemically configured hydrogels are presently preferred for use in the invention. In one, an oxidative enzyme is immobilized within a pH-sensitive hydrogel, and catalyzes a reaction of the analyte to produce a charged product. The term xe2x80x98pH-sensitive hydrogelxe2x80x99 refers generally to a hydrogel modified to contain pendant charged groups in proportions that produce an overall acidic or basic environment in the fluid within the gel. The immobilized enzyme might be, for example, glucose oxidase, GOX, where the analyte to be measured is glucose. The charged product generated by activity of the enzyme on the analyte causes the hydrogel to change its displacement volume (swell or shrink), which changes can be detected by the displacement detection means. The second type of hydrogel has both analyte binding molecules (ABMs) and analyte or analyte analogue molecules (AAMs) co-immobilized within it, in addition to charged pendant groups. In the absence of free analyte, immobilized ABMs bind to immobilized AAMs, forming what are in effect non-covalent xe2x80x98crosslinksxe2x80x99. As free analyte from a body fluid or test solution diffuses into the hydrogel, binding competition displaces immobilized AAMs from ABMs, thus reducing the number of xe2x80x98crosslinksxe2x80x99. This reduction in crosslinking causes swelling of the hydrogel.
Also, in addition to the above two types of hydrogels, it is within contemplation that other analyte-sensitive swellable materials, polymers, and hydrogels meeting that description may be developed and will be useful in the biosensor. Certain embodiments of the invention are specifically designed to detect glucose levels in body fluids.
In its broadest conception, the invention is an implantable analyte (for example, glucose) biosensor containing an analyte-sensitive hydrogel filament and a photometric displacement transducer. The displacement of the hydrogel filament changes with changes in the concentration of the analyte. A set of a light source and light intensity detector (photoreceptor) measures the displacement of the hydrogel by detecting changes of intensity of light that falls on the detector: the intensity of light received is converted to an electric signal. In a preferred embodiment, a photo diode and a phototransistor are the light source and the light intensity detector, respectively. Such a biosensor comprises a rigid biocompatible enclosure having one or more openings permitting penetration of a patient""s body fluid to the hydrogel. The hydrogel filament is preferably disposed within a column in the enclosure and is configured to swell freely in only one dimension relative to a fixed end.
The hydrogel is chemically configured to vary its displacement volume according to changes in concentration of the particular analyte to be measured, such as glucose, in the body fluid, and is positioned, such as between the light source and the light intensity detector, so that the light falling on the light intensity detector is determined by the displacement of the hydrogel.
If desired, the biosensor and the hydrogel can be fabricated in a micro level. Brock et al. describe attempts to make artificial muscle from bundles of extremely thin (10 micron diameter) polyacrylonitrile fibers (xe2x80x9cDynamic Model of Linear Actuator based on Polymer Hydrogelxe2x80x9d, published on the Web at www.ai.mit.edu/projects/muscle/papers/icim94). In addition, modem optical MEMS technology permits the photometric devices such as a photo LED and a photo detector to be fabricated in a micro level. Those who are skilled in these fields can accommodate a miniaturized photometric glucose biosensor based on the proposed idea and the up-to-date technology.
The invention further encompasses methods of determining the concentration of free analyte in a solution and of making the biosensor. The method of determining analyte concentration comprises steps of: providing a hydrogel in a manner so that the displacement of the hydrogel changes depending upon the concentration of the analyte being measured and detecting the displacement of the hydrogel using a photometric detector where light sensed by a photoreceptor indicates hydrogel displacement. Further, the method may include providing a hydrogel having pendant charged and/or uncharged moieties, analyte molecules, and analyte-specific binding molecules covalently immobilized therein; contacting the hydrogel sequentially with a series of calibration solutions having known concentrations of free analyte, and measuring the displacement of the hydrogel for each of the calibration solutions to produce a calibration curve of displacement versus analyte concentration; contacting the hydrogel with the test fluid, and measuring a resulting displacement; and comparing the resulting displacement with the calibration curve to determine the analyte concentration of the test fluid. A further embodiment of the method includes a step of enclosing the hydrogel in a rigid structure which has at least one permeable portion through which free analyte in the test solution can diffuse into the hydrogel, with the structure sized and configured to permit hydrogel expansion in substantially only one dimension. Limiting the hydrogel expansion and contraction to substantially only one dimension makes the measurement of displacement more sensitive.
A further embodiment of the biosensor includes reporting means associated with the displacement detection means for reporting a data signal reflective of hydrogel displacement, and computing means operably disposed to receive the data signal and constructed to compare it to a predetermined limit and to produce a warning or alarm notification if at that predetermined limit. In a preferred embodiment, the reporting means is a battery-powered telemeter that sends a radio data signal to a receiver operably attached to the computing means. In a further preferred embodiment, the computing means is associated with an alarm system. The computing means may be a personal computer, but in a preferred embodiment, the computing means is a microprocessor. In a more highly preferred embodiment, the computing means contains or is operably associated with alarm means for providing an alarm signal when the analyte concentration falls outside a pre-determined acceptable range. In a further highly preferred embodiment, the biosensor unit carried in or on the patient""s body includes a GPS (global positioning system) unit.
Thus, a further invention described herein comprises biosensor-based health alarm system which provides a warning of an adverse condition detected by a biosensor to care providers at a location remote to the patient via telephone or wireless transmission means. In a highly preferred embodiment, the system includes a GPS unit and a wireless phone, thus providing monitoring and alarm coverage to the patient while traveling. The biosensor of the system may be any sensor configured to detect a critical health-related biological determinant (such as, but not limited to, the concentration of a selected analyte, such as glucose in the patient""s body fluid). The system may further include an automatic drug administration component that responds to the sensor by administering an appropriate amount of a drug to ameliorate the adverse effects of the change in the biological determinant.